Hello, acmatos. I think I can help (I'm using the same French dictionary)
I'm attaching a small application I made that can patch the language of a mobi book. Please use it only on a backup copy of the file you want to modify, it's an application made in an hour or so!
The idea is that for the books I have I could change the language code so I could play with it (in the beginning I thought the languages of the book being read and the default dictionary must match, but it's not the case with Kindle). After a while I have realised that the only good thing this dirty app can be used for is to patch a dictionary file to English... But if it will help you, that's all that matters.
It's easy to use:
1. Make a copy of your file (the MOBI dictionary).
2. Open that copy.
3. Select English (US).
3. Click Patch and wait (it should take less than a minute).
Done!
I hope it is working...